Current sensors

ABSTRACT

A current sensor comprises an outer shielding means ( 4 ), a toroidal inner core and inner shield means ( 1 ). A combined driving and sensing winding ( 3 ) is wound around the inner core. First and second ends of the winding ( 3 ) are connectable to a sensor head. The inner shield means ( 1 ) has a gap ( 1 A) formed in it.

[0001] The invention relates to current sensors.

[0002] In the prior art, there are described many different ways fornon-invasively measuring current flowing in a conductor. One well knownmethod is to use what is known as a direct current transformer whichtypically uses a relatively low frequency excitation current and using atwo-core method. In such a two-core method, a DC current through the twocores biases each with flux at the same polarity, while the flux in onecore due to a modulator drive remains out of phase with that in thesecond. Each core reaches a saturation flux level so that it reachessaturation flux level at a different point in the excitation cycle forone alternation than for the other alternation. The result is an outputsignal at even harmonics of the excitation frequency.

[0003] Techniques such as the above are used to provide an absolutemeasurement of current. In many cases however, such as monitoringinsulation breakdown in high current conductors, it is required tomeasure leakage currents in the presence of very high drive currents.For instance, DC motors on locomotives might have a drive current of3000 amps and the leakage current desired to be monitored for insulationbreakdown may well be in the order of milliamps. In such cases, thelarge drive currents result in very large magnetic fields and, eventhough send and return conductors may carry almost the same current, itis clear that the fringing magnetic fields which in this case will notbe insignificant, would not cancel and will be dependent on the positionof the conductors within the cores. There would therefore result apositionally dependent measurement and no coherent information would berecoverable. Some manufacturers have sought to obviate these problems byutilising complicated bus bar geometry so as to cancel out the fringingmagnetic fields. However, apart from being costly such solutions areoften not practical.

[0004] It is a first aim of embodiments of the present invention toprovide a novel current sensing apparatus which allows the measurementof small leakage currents in the presence of high drive currents withoutrequiring complicated bus bar geometry.

[0005] It is a further aim of embodiments of the present invention toprovide a current sensor geometry in which only a single measurementcore is required.

[0006] It is a still further aim to provide means for monitoringinsulation breakdown in conductors.

[0007] According to a first aspect of the invention, there is provided acurrent sensor comprising an outer shielding means, a toroidal innercore and inner shield means, there being further provided a combineddriving and sensing winding being wound around said inner core, firstand second ends of said winding being connectable to a sensor head, saidinner shield means having a gap formed therein.

[0008] Preferably, the inner shield means is located in a region insidean inner diameter of the inner core. Said inner shield means preferablycomprises a ring of material.

[0009] The ring of material may be of, for instance, a magnetic steel.The inner ring may be laminated.

[0010] Preferably, a cover piece is provided for supporting the magneticsensor. Preferably, the cover piece is of a non-conductive, preferablyplastics, construction.

[0011] Preferably, the outer shielding means comprises an outer coreforming an outer periphery of said current sensor. The outer core maycomprise a series of laminations.

[0012] Preferably, the outer core is provided with a gap formed therein.

[0013] Preferably, the gap in the outer core is not aligned with the gapin the inner core.

[0014] Preferably, the laminations comprise a magnetic steel.

[0015] Preferably, the inner core comprises high performanceferromagnetic material.

[0016] The inner core may comprise an amorphous ferromagnetic alloy.

[0017] The inner core may comprise a high nickel ferromagnetic alloy.

[0018] Preferably, said winding has a first end which is arranged toreceive a drive signal and a second end which is arranged for providingan output sensing signal. Preferably, the second end is connected to asense resistor.

[0019] Preferably, the first end is connected to a source of excitationat a frequency F. Preferably, the second end is connected to a band passfilter centred at a frequency of 2F. Preferably, the frequency F issubstantially 4 KHz.

[0020] Preferably, the output of the band pass filter is thereafterrectified and integrated.

[0021] Preferably, the sensor winding is excited by means of a source ofexcitation applied to a first, inverting, input of a differentialamplifier. Preferably, the second, non-inverting, input of thedifferential amplifier is connected to earth and the output of thedifferential amplifier being connected to the first end of the winding.

[0022] Preferably, the rectified band pass filtered and integratedoutput is fed back to the inverting input of the differential amplifiervia a feed back resistor.

[0023] The integrated rectified band pass filtered output may be furtherconnected to processing circuitry. The processing circuitry may includedata logging circuitry. The data logging circuitry may be arranged tomonitor occurrences of non-zero outputs against time.

[0024] If the number of non-zero outputs within a given time periodexceeds a threshold value, then the data logging circuitry may output awarning signal.

[0025] The warning signal may give notice that an unacceptable level ofleakage current has occurred within a given time period.

[0026] Such data logging circuitry may be useful in monitoringinsulation breakdown in conductors.

[0027] According to a second aspect of the invention, there is provideda clamp on probe comprising a first limb and a second limb, the firstand second limbs being shaped so as to cooperate with one another toform a substantially complete magnetic circuit when the probe is in aclosed configuration, the probe being characterised by the followingfeatures: said first and second limbs comprise ferromagnetic materialwhich act as a flux guide when said probe surrounds a current carryingconductor, said first and second limbs each being provided with a recessin their inner periphery in which first and second current sensor meansare housed, said first and second recesses being arranged so as toshield said first and second current sensing means from field effectscaused by the earth's magnetic field.

[0028] Preferably, said first and second current sensors are connecteddifferentially so as to respond to a differential magnetic fieldproduced by the conductor around which the probe is fitted.

[0029] Preferably, said first and second current sensors comprise linearflux gates.

[0030] Preferably, said first and second sensors are driven, in use, ata high frequency, preferably in the range of 80 to 200 KHz.

[0031] Preferably, the two sensors are driven in parallel at a frequencyof substantially 120 KHz.

[0032] Preferably, each sensor is connected in series with a capacitorwhich is tuned to resonance.

[0033] Preferably, output voltages across the two capacitors are fed toa differential amplifier and subtracted.

[0034] According to a third aspect of the invention, there is provided afuse monitor, the monitor comprising a fuse holder for receiving a fuse,and an electric field strength sensor, wherein the electric fieldstrength sensor is disposed so as to capacitively charge in response tocurrent flow through the fuse.

[0035] Preferably, the electric field strength sensor is substantiallyplanar and arranged such that one face is directed towards a position inwhich the fuse is, in use, received.

[0036] Said electric field strength sensor may comprise a foil.

[0037] Preferably, the electric field strength sensor is connected viasignal processing means to an alarm indicator arranged to displaywhether the said fuse is intact or blown.

[0038] The signal processing means may comprise a high input impedancevoltage follower.

[0039] The signal processing means may comprise a comparator circuit forcomparing an electrostatically induced voltage of the electric fieldstrength sensor with a reference voltage.

[0040] The fuse monitor may further comprise a current sensor formonitoring, in use, current flow through a fuse received by the fuseholder.

[0041] Preferably, the current sensor is substantially toroidal andarranged, in use, to circumferentially surround the fuse in the fuseholder such that a conductor comprising said fuse extends through theaperture defined by the toroidal nature of the current sensor and insubstantially parallel relation to a central axis of said currentsensor.

[0042] The current sensor is preferably a sensor in accordance with thefirst aspect of the invention.

[0043] For a better understanding of the invention, and to show howembodiments of the same may be carried into effect, reference will nowbe made, by way of example, to the accompanying diagrammatic drawings inwhich:

[0044]FIG. 1 is a schematic electrical circuit diagram illustrating acurrent sensor in accordance with a first embodiment of the invention;

[0045]FIG. 2 is a plan external view of the sensor of FIG. 1;

[0046]FIGS. 3A to C show a plastics cover piece for use with the sensorof FIGS. 1 and 2;

[0047]FIG. 4 illustrates a circuit for driving and sensing the output ofthe current sensor of FIGS. 1 and 2;

[0048]FIG. 5 is a view of a first limb of a probe according to thesecond aspect of the invention;

[0049]FIG. 6 is a view showing a second limb of a probe in accordancewith the second-aspect;

[0050]FIG. 7 is a view showing a fuse and fuse holder including acurrent sensor in accordance with the first aspect and furtherincorporating an electric field strength sensor;

[0051]FIG. 8 illustrates the positioning of the electric field sensor inrelation to the fuse and fuse holder of FIG. 7;

[0052]FIG. 9 illustrates schematically how the electric field sensor maybe connected to provide an output indicative of the integrity of a fuse;

[0053]FIG. 10 is an equivalent circuit of the schematic arrangementshown in FIG. 9; and

[0054]FIG. 11 illustrates how such an electric field sensor may becombined with external circuitry to provide a voltage alarm.

[0055] Referring now to FIG. 1, there is shown a current sensor inaccordance with the first aspect of the invention.

[0056] The current sensor probe comprises an inner shield 1, a coverpiece 2, a toroidal winding 3 and an outer shielding means comprising anouter shield means wound on an inner core.

[0057] The inner shield 1 is shaped as an incomplete ring and includes agap 1A separating end faces of the ring. The gap is small in comparisonto the circumference of the inner shield 1 and in typical applicationsmay be of the order of 1 or 2 mm, whilst the circumference of the innershield 1 may be in typical applications around 30 cm.

[0058] The toroidal winding 3 is formed on an inner core comprising amaterial having a substantially square B-H loop with high intrinsicpermeability. Such a material may be, for instance, nickel iron or anamorphous ferromagnetic alloy.

[0059] The outer shield 4 shields at least an outer periphery of thewinding 3 from external field effects and may comprise a series ofmagnetic steel laminations. The laminations forming the outer shield 4do not form a complete ring, but instead are separated by a small gap 40at adjacent end faces. This gap may typically be around 2 mm, thepurpose of the outer core is to confine all fringing fields and givepositional independence to the sensor. In this way, with send andreceive cables running inside the inner periphery of the probe andsurrounded by the probe, the measurement result is independent ofproximity of those send and receive cables as both of their fringingfields will be confined to the outer core and will sum to zero whenthere is no leakage current present. The purpose of the gap 40 is toincrease the saturation level of the outer core for a given excitation.

[0060] Referring now to FIG. 2, there is shown in plan view the externalappearance of a magnetic sensor as described in relation to FIG. 1. Ascan be seen, the outer shield 4 is built up of a number of laminationsand features gap 40. Inside the outer shield, there is the toroidalwinding 3 formed on the inner core. Then, concentrically arranged insideof the toroidal winding 3 there is the inner shield 1, with gap 1A.

[0061] Referring now to FIGS. 3A to 3C, there is shown a cover piecewhich holds the arrangement of FIG. 2. The cover piece 2 is shown inbottom plan view in FIG. 3A, in top plan view in FIG. 3C and insectional view in FIG. 3B. The cover piece 2 is a stepped plasticsmember having a first, larger, inner diameter D which is arranged to bethe same as the outer diameter of the outer shield 4 shown in FIG. 2. Asecond, inner diameter d of the cover piece 2 is arranged to be the sameas the inner diameter of the inner shield 1. Therefore, as can be seen,the magnetic sensor arrangement of FIG. 2 may be simply placed into theplastics cover piece 2 and, in use, the conductors which are to bemonitored with regard to current detection may be fed into a mouthregion M (shown in FIG. 3B) and fed through the sensor to pass withinthe inner diameter of the inner shield 1.

[0062]FIG. 4 shows a circuit which may be used in combination with thecurrent sensor of FIGS. 1 and 2. In the circuit, there is shown adifferential amplifier 5, having its non-inverting input grounded andits inverting input connected to a source 6 of excitation at 4 KHz via abias resistor R_(B). The inverting input is also connected to a feedbackloop via a feedback resistor R_(F). The output of the differentialamplifier 5 is connected to the sensor winding 3 to provide drive to theone end of the winding 3, the other end of the winding 3 being connectedvia a sense resistor R_(s) to earth. The junction of the sense resistorR_(s) and the winding 3 is connected to the input of a band pass secondharmonic filter 8, centred at 8 KHz. The output of the filter 8 is fedto a rectifier 9, whose output is then fed to an integrator 10 and theintegrated rectified second harmonic output is thereafter fed toprocessing circuitry and (typically) display and/or data loggingcircuitry (not shown). The output is thereafter fed back through thefeed back resistor R_(F) to the inverting input of the differentialamplifier 5.

[0063] In a sensor of the type described in relation to FIGS. 1 and 2,there is a linear relationship between the second harmonic voltagesignal and the external magnetic field. If the sensor is excitedasymmetrically, then it produces even harmonics. It is this evenharmonic, the second harmonic at 8 KHz, that is therefore demodulated,integrated and used as the controlling voltage to close the feedbackloop. The control voltage is an indication of the strength of themagnetic field and, thus, the concommitant current.

[0064] As mentioned previously, when there are no leakage currentsbecause of the geometry of the current sensor of FIG. 1, currentsflowing within the core will sum to zero. However, when there is aleakage current, such currents will produce a non-zero core responsewhich, by virtue of the gap 1A causes the current sensor of FIG. 2 toproduce an output which directly relates to that leakage current andenables very small currents to be measured in the presence of very highdrive currents.

[0065] Leakage currents give a measure of the integrity of conductorinsulation as where there is insulation breakdown, leakage currentsoccur. In this regard, the sensor of FIGS. 1 and 2 and the circuit ofFIG. 4 can be connected to a data logger so as to monitor the occurrenceof such leakage currents over time. In this way, the condition ofinsulation can be monitored to allow timely replacement beforedeterioration progresses beyond an acceptable level.

[0066] Referring now to FIGS. 5 and 6, there is shown another aspect ofthe present invention. FIG. 5 shows a first, upper limb of a currentsensing probe and FIG. 6 shows a second, lower limb of the same probe.

[0067] In FIG. 5, the upper limb comprises a core 11, having a recess 12formed therein. In the recess 12, there is provided a linear flux gate13. Similarly, in FIG. 6, the lower limb comprises a core 14, having arecess 15 formed therein and a linear flux gate 16 situated within therecess 15.

[0068] In use, the upper and lower limbs of the probe of FIGS. 5 and 6form first and second jaws of a clamp-on probe. Traditional clamp-oncurrent sensing probes utilise Hall sensors for detecting the magneticfield resulting from an electric current passing through a conductor.These sensors are, in the main, insensitive to electric currents lessthan 10 mAmps. There is a market requirement for a probe to measure suchvalues.

[0069] The positioning of the flux gates 13, 16 within recesses 12, 15is done so as to reduce earth field effects.

[0070] The two flux gate sensors 13, 16 are connected differentially(not shown) so as to respond to a differential magnetic field producedby a conductor.

[0071] The flux gate sensitivity is dependent upon a number of factors:the number of turns, length of core material and the coupling betweenthe coil and the core material.

[0072] The two sensors are driven in parallel at a frequency of 120 Khz.Each sensor is in series with a capacitor (not shown) tuned toresonance. The output voltages across the two capacitors are fed to adifferential amplifier and subtracted. The reason for this is that thetwo sensors do not attain saturation simultaneously, because they areboth exposed to different external earth field values. Consequently, thetwo second harmonic signals are not in phase. It can thus be seen thatif these two signals are subtracted, then the carrier signals willcancel out and only the second harmonic signals will remain. Thesesecond harmonic signals are then rectified on both the positive andnegative peaks and the resulting DC offsets are summed to zero throughan integrator. Circuitry similar to that shown in FIG. 4 can be adaptedfor such use.

[0073] The phase shift is detected as a DC offset change, because thesystem comprises a carrier that is both phase and amplitude modulated.The integrator drives a power output stage which drives a compensationcoil, resulting in the cancellation of the sampled field.

[0074]FIG. 7 shows a further embodiment of the present invention and inparticular shows a fuse holder including a current sensor along withelectric field strength sensing means.

[0075] Considering FIG. 7 in detail, there is shown, a housing 70 and afuse carrier 72. The fuse carrier further includes fuse holding brackets74A, 74B in which a fuse 76 is insertable, a current sensor 77 throughwhich the fuse 76 is insertable and which is arranged, in use, tocircumferentially surround the fuse 76. There is also provided anelectric field strength sensor 78 built into a wall of the fuse carrier72.

[0076] The current sensor 77 is preferably of the type discussedearlier. There is now described the operation of the electric fieldstrength sensor 78 and this is described with the aid of FIGS. 8 to 11(in which, for clarity, the current sensor 77 is omitted).

[0077] Referring to FIG. 8, there is shown a fuse 76, electric fieldstrength sensor 78 and fuse holding brackets 74A, 74B. There is alsoshown in schematic form a nominal load represented by load resistorR_(load) and a current path (I) which in accordance with conventionflows in a positive direction from a voltage source V, through the fuse76 and into a load R_(load).

[0078] The electric field strength sensor 78 may, conveniently, comprisea capacitive or electro-static inductive sensor, perhaps being a simplepiece of foil, such as a thin aluminium foil. This electric fieldstrength sensor 78 effectively charges up capacitively in accordancewith the current I flowing through the fuse. Indeed, the sensor 78should be mounted as close to the fuse as possible so that one side ofthis piece of foil is directed towards the conductor comprising the fuse76. This electric field strength sensor 78 is also preferably shieldedfrom other external electrical fields so that the charge on the electricfield strength sensor 78 varies substantially in accordance with thecurrent I flowing through the fuse 76. This shield is representedschematically as an element 70 in FIG. 9.

[0079] As is shown schematically in FIG. 9, the piece of foil comprisingthe electric field strength sensor 78 is shielded by earthed element 79to minimise external field effects and the sensor 78 is connected to ahigh input impedance voltage follower VF. The shield 79 is insulatedfrom the sensor 78, for instance, by being glued to one side of it by anon-conductive glue. The voltage follower VF must have a very high inputimpedence in order to avoid loading the electric field strength sensor78. In this way, bias currents of the amplifier comprising VF must bevery small, otherwise the electrostatically induced charge on the sensor78 will leak away to contribute to the bias current. Also, inputcapacitance of the amplifier must be low.

[0080]FIG. 10 shows the equivalent circuit of FIG. 9, with the sensor 78being represented by capacitance C_(sensor).

[0081] Referring now to FIG. 11, there is shown a voltage alarm circuitfor indicating whether or not the fuse 76 is intact. The circuitry ofFIG. 11 comprises sensor 78 represented by C_(sensor), voltage followerVF as described in relation to FIG. 10, the output of voltage followerVF being input to the non-inverting input of an amplifier AMP1,amplifier AMP1 being connected as a comparator so that a referencevoltage V_(ref) appears at its inverting input, V_(ref) being determinedby biasing resistors R_(B1) and R_(B2) which are connected in seriesbetween a supply voltage V_(supply) and earth. The inverting input ofthe amplifier AMP1 is connected the common connection between seriesconnected bias resistors R_(B1), R_(B2) and the output of amplifier AMP1is connected to an output pull-up resistor R_(out).

[0082] The operation of FIG. 11 will now be described in two cases, inthe first case the fuse 76 being intact, and in the second case, thefuse 76 being blown.

[0083] In the first case, where a current I flows through fuse 76, oneside of the piece of foil comprising the electric field strength sensor78 charges electrostatically as a consequence of the presence of currentI. Because the sensor 78 reacts in a capacitive manner, the potentialdifference between the fuse electrode and the piece of foil comprisingthe sensor quickly increases and stabilises at a particular voltageV_(sensor). In this first case, the voltage V_(sensor) is buffered bythe voltage follower VF and input to the non-inverting input of AMP1.This voltage is compared with V_(ref) and, if V_(sensor) is higher thanV_(ref) then AMP1 will output a non-zero voltage indicative of the factthat the fuse 76 is intact.

[0084] In the second case, where the fuse 76 is blown any charge on theelectric field strength sensor 78 will dissipate and V_(sensor) willreduce. Once V_(sensor) has dropped below the level of V_(ref), then thecomparator formed by AMP1 will switch states so as to no longer output apositive voltage. With this switching of states, the output of AMP1 maybe used to drive, for instance, an LED display indicating that the fuse76 has blown.

[0085] The reader's attention is directed to all papers and documentswhich are filed concurrently with or previous to this specification inconnection with this application and which are open to public inspectionwith this specification, and the contents of all such papers anddocuments are incorporated herein by reference.

[0086] All of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), and/or all of the stepsof any method or process so disclosed, may be combined in anycombination, except combinations where at least some of such featuresand/or steps are mutually exclusive.

[0087] Each feature disclosed in this specification (including anyaccompanying claims, abstract and drawings), may be replaced byalternative features serving the same, equivalent or similar purpose,unless expressly stated otherwise. Thus, unless expressly statedotherwise, each feature disclosed is one example only of a genericseries of equivalent or similar features.

[0088] The invention is not restricted to the details of the foregoingembodiment(s). The invention extend to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

1. A current sensor comprising an outer shielding means, a toroidalinner core and inner shield means, there being further provided acombined driving and sensing winding being wound around said inner core,first and second ends of said winding being connectable to a sensorhead, said inner shield means having a gap formed therein.
 2. A currentsensor as claimed in claim 1, wherein the inner shield means is locatedin a region inside an inner diameter of the inner core.
 3. A currentsensor as claimed in claim 1 or claim 2, wherein said inner shield meanscomprises a ring of material.
 4. A current sensor as claimed in claim 3,wherein the ring of material comprising a magnetic steel.
 5. A currentsensor as claimed in claim 3 or claim 4, wherein the ring is laminated.6. A current sensor as claimed in any one of the above claims, thesensor further comprises a cover piece for supporting a magnetic sensor.7. A current sensor as claimed in claim 6, wherein the cover piece is ofa non-conductive, preferably plastics, construction.
 8. A current sensoras claimed in any one of the above claims, wherein the outer shieldingmeans comprises an outer core forming an outer periphery of said currentsensor.
 9. A current sensor as claimed in claim 8, wherein the outercore comprises a series of laminations.
 10. A current sensor as claimedin claim 8 or claim 9, wherein the outer core is provided with a gapformed therein.
 11. A current sensor as claimed in claim 10, wherein thegap in the outer core is not aligned with the gap in the inner core. 12.A current sensor as claimed in any one of claims 8 to 11, wherein thelaminations comprise a magnetic steel.
 13. A current sensor as claimedin any one of the above claims, wherein the inner core comprises atleast one of high performance ferromagnetic material; an amorphousferromagnetic alloy; and a high nickel ferromagnetic alloy.
 14. Acurrent sensor as claimed in any one of the above claims, wherein saidwinding has a first end which is arranged to receive a drive signal anda second end which is arranged for providing an output sensing signal.15. A current sensor as claimed in claim 14, wherein the second end isconnected to a sense resistor.
 16. A current sensor as claimed in claim14 or claim 15, wherein the first end is connected to a source ofexcitation at a frequency F.
 17. A current sensor as claimed in any oneof claims 14 to 16, wherein the second end is connected to a band passfilter centred at a frequency of 2F.
 18. A current sensor as claimed inclaim 17, wherein the frequency F is substantially 4 KHz.
 19. A currentsensor as claimed in claim 17 or claim 18, wherein the output of theband pass filter is thereafter rectified and integrated.
 20. A currentsensor as claimed in any one of the above claims, wherein the sensorwinding is excited by means of a source of excitation applied to afirst, inverting, input of a differential amplifier.
 21. A currentsensor as claimed in claim 20, wherein a second, non-inverting, input ofthe differential amplifier is connected to earth and an output of thedifferential amplifier is connected to the first end of the winding. 22.A current sensor as claimed in claim 21, wherein the rectified band passfiltered and integrated output is fed back to the inverting input of thedifferential amplifier via a feed back resistor.
 23. A current sensor asclaimed in claim 22, wherein the integrated rectified band pass filteredoutput is further connected to processing circuitry.
 24. A currentsensor as claimed in claim 23, wherein the processing circuitry includesdata logging circuitry.
 25. A current sensor as claimed in claim 24,wherein the data logging circuitry is arranged to monitor occurrences ofnon-zero outputs against time.
 26. A current sensor as claimed in claim25, wherein circuitry is arranged to output a warning signal if thenumber of non-zero outputs within a given time period exceeds athreshold value.
 27. A current sensor as claimed in claim 26, whereinthe warning signal is arranged to give notice that an unacceptable levelof leakage current has occurred within a given time period.
 28. A clampon probe comprising a first limb and a second limb, the first and secondlimbs being shaped so as to cooperate with one another to form asubstantially complete magnetic circuit when the probe is in a closedconfiguration, the probe being characterised by the following features:said first and second limbs comprise ferromagnetic material which act asa flux guide when said probe surrounds a current carrying conductor,said first and second limbs each being provided with a recess in theirinner periphery in which first and second recesses being arranged so asto shield said first and second current sensing means from field effectscaused by the earth's magnetic field.
 29. A probe as claimed in claim28, wherein said first and second current sensors are connecteddifferentially so as to respond to a differential magnetic fieldproduced by the conductor around which the probe is fitted.
 30. A probeas claimed in claim 28 or claim 29, wherein said first and secondcurrent sensors comprise linear flux gates.
 31. A probe as claimed inany one of claims 28 to 30, wherein said first and second sensors aredriven, in use, at a high frequency.
 32. A probe as claimed in claim 31,wherein said sensors are driven in the range of 80 to 200 KHz.
 33. Aprobe as claimed in claim 32, wherein the two sensors are driven inparallel at a frequency of substantially 120 KHz.
 34. A probe as claimedin any one of claims 28 to 33, wherein each sensor is connected inseries with a capacitor which is tuned to resonance.
 35. A probe asclaimed in claim 34, wherein output voltages across the two capacitorsare fed to a differential amplifier and subtracted.
 36. A fuse monitor,the monitor comprising a fuse holder for receiving a fuse, and anelectric field strength sensor, wherein the electric field strengthsensor is disposed so as to capacitively charge in response to currentflow through the fuse arranged such that one face is directed towards aposition in which the fuse is, in use, received.
 38. A fuse monitor asclaimed in claim 36 or claim 37, wherein said electric field strengthsensor comprises a foil.
 39. A fuse monitor as claimed in any one ofclaims 36 to 38, wherein the electric field strength sensor is connectedvia signal processing means to an alarm indicator arranged to displaywhether said fuse is intact or blown.
 40. A fuse monitor as claimed inclaim 39, wherein the signal processing means comprises a high inputimpedance voltage follower.
 41. A fuse monitor as claimed in claim 39 orclaim 40, wherein signal processing means comprises a comparator circuitfor comparing an electrostatically induced voltage of the electric fieldstrength sensor with a reference voltage.
 42. A fuse monitor as claimedin any one of claims 36 to 41, wherein the fuse monitor furthercomprises a current sensor for monitoring, in use, current flow througha fuse received by the fuse holder.
 43. A fuse monitor as claimed inclaim 42, wherein the current sensor is substantially toroidal andarranged, in use, to circumferentially surround the fuse in the fuseholder such that a conductor comprising said fuse extends through theaperture defined by the toroidal nature of the current sensor and insubstantially parallel relation to a central axis of said currentsensor.
 44. A fuse monitor as claimed in any one of claims 36 to 43,wherein the current sensor is a sensor in accordance with claim 1 or anyclaim dependant thereto.